1. Field of the Invention
The present invention relates to a liquid crystal display device in which a display medium having liquid crystal regions partitioned by polymer walls formed of a polymer is sandwiched between two facing substrates, and a method for producing the same.
2. Description of the Related Art
There are various kinds of display modes for a liquid crystal display device. For example, as liquid crystal display devices taking advantage of electro-optic effects, liquid crystal display devices of a twisted nematic (TN) mode, a super twisted nematic (STN) mode, etc. using nematic liquid crystal molecules have been put to practical use. In addition, a liquid crystal display device using ferroelectric liquid crystal (FLC) has been proposed. These liquid crystal display devices require polarizing plates and an orientation treatment.
Examples of liquid crystal display devices taking advantage of light scattering of liquid crystal, requiring no polarizing plates include a liquid crystal display device of a dynamic scattering (DS) mode or a phase change (PC) mode.
In recent years, a liquid crystal display device requiring no polarizing plates or orientation treatment; i.e., a liquid crystal display device taking advantage of the birefringence of liquid crystal and electrically regulating a transparent state and an opaque state of the device has been proposed. According to the system of this type of liquid crystal display device, a display is performed as follows:
When liquid crystal molecules are aligned by the application of a voltage, the ordinary refractive index of the liquid crystal molecules and the refractive index of a medium for supporting the liquid crystal molecules, such as a polymer coincide with each other, whereby a transparent state is obtained. On the other hand, when no voltage is applied to the device, light scattering is caused due to the turbulence of the orientation of the liquid crystal molecules. In this way, a display is performed.
As a method for producing a light scattering type liquid crystal display device of this system, the following five methods have been proposed.
(1) A display medium is obtained by making liquid crystal contained in polymer capsules (Japanese National Publication No. 58-501631). PA1 (2) A light-curable or thermosetting resin and liquid crystal are mixed; and only the resin is deposited and cured. Then, liquid crystal regions in a spherical shape are formed in the cured resin (Japanese National Publication No. 61-502128). PA1 (3) The diameter of spherical liquid crystal regions is regulated (Japanese Laid-Open Patent Publication No. 3-72317). PA1 (4) A polymer porous film is impregnated with liquid crystal (Japanese Laid-Open Patent Publication No. 3-59515). PA1 (5) Beads formed of a polymer which become a source for light scattering are floated in liquid crystal provided between two transparent electrodes disposed apart from each other (Japanese Laid-Open Patent Publication No. 3-46621). PA1 two substrates facing each other, at least one of the substrates being transparent; PA1 electrodes disposed on inside surfaces of the respective substrates; PA1 a display medium which is provided between the two substrates and formed of polymer walls and liquid crystal regions partitioned by the polymer walls; and PA1 a plurality of pixels, PA1 wherein an interval a between a center of one liquid crystal region and a center of an adjacent liquid crystal region in a direction along a surface of the substrate is within a width of one pixel along the direction, and 80% or more of the intervals a satisfy the relationship: 3b/2&gt;a&gt;b/2, where b is an average of the intervals a. PA1 providing a mixture containing a photopolymerizable compound and a liquid crystal material between a pair of substrates, two substrates facing each other, at least one of the substrates being transparent, electrodes being disposed on inside surfaces of the respective substrates; and PA1 irradiating light to the mixture with a light intensity distribution in which light intensity of at least one portion of each pixel is 90% or less of a maximum illuminance in a circular area which corresponds to 10 times the pixel area and whose center is situated in a center of the pixel. PA1 two substrates facing each other, at least one of the substrates being transparent, electrodes disposed on inside surfaces of the respective substrates; and PA1 a display medium which is provided between the two substrates and formed of polymer walls containing a polymer as their main component and liquid crystal regions containing liquid crystal as their main component; PA1 wherein the liquid crystal regions are partitioned by the polymer walls and are close to the substrates, portions of the liquid crystal regions close to the substrates being in parallel with the substrates. PA1 providing a mixture containing a photopolymerizable compound and a liquid crystal material between a pair of substrates, two substrates facing each other, at least one of the substrates being transparent, and electrodes being disposed on inside surfaces of the respective substrates, thereby forming a cell; and PA1 irradiating the mixture with light under the condition that intensity of light is reduced in predetermined portions of the mixture, thereby forming a display medium between the substrates, the display medium having polymer walls containing a polymer as their main component and liquid crystal regions containing liquid crystal as their main component. PA1 forming an orientation film containing a photopolymerization initiator on at least one of a pair of substrates, two substrates facing each other, at least one of the substrates being transparent, and electrodes being disposed on inside surfaces of the respective substrates; PA1 subjecting the substrate on which the orientation film is formed to a rubbing treatment in one direction; PA1 providing a mixture containing a photopolymerizable compound and a liquid crystal material between the pair of substrates after the rubbing treatment; and PA1 forming a display medium having polymer walls containing a polymer as their main component and liquid crystal regions containing liquid crystal as their main component by curing the photopolymerizable compound. PA1 forming a thin film pattern containing a photopolymerization initiator on one surface of at least one of a pair of substrates, the substrates respectively having electrodes and at least one of the substrates being transparent; PA1 providing a mixture containing a polymerizable compound and a liquid crystal material between the pair of substrates, at least one of the substrates having the thin film pattern, thereby forming a cell; and PA1 forming a display medium between the substrates by curing the polymerizable compound, the display medium having polymer walls containing a polymer as their main component and liquid crystal regions containing liquid crystal as their main component. PA1 providing a mixture between a pair of substrates facing each other, thereby forming a cell, at least one of the substrates being transparent, electrodes being disposed on inside surfaces on the respective substrates, the mixture containing a liquid crystal material, a polymerizable liquid crystalline material having a liquid crystalline functional group in its molecule, a polymerizable compound, and a polymerization initiator, anisotropy of dielectric constant .DELTA..epsilon..sub.L of the liquid crystal material and anisotropy of dielectric constant .DELTA..epsilon..sub.p of the polymerizable liquid crystalline material having a relationship of .DELTA..epsilon..sub.L .times..DELTA..epsilon..sub.p &lt;0, and PA1 forming a display medium between the substrates by polymerizing the polymerizable compound, the display medium having polymer walls containing a polymer as their main component and liquid crystal regions containing liquid crystal as their main component, providing the liquid crystalline functional groups in the liquid crystal regions to fix a liquid crystalline polymer on the polymer walls. PA1 coating the mixture onto one of the substrates, the mixture further containing a solvent capable of homogeneously dissolving the liquid crystal material and the polymerizable liquid crystalline material; PA1 removing the solvent from the mixture coated onto one of the substrates by evaporation to provide the liquid crystalline functional groups in the liquid crystal regions, thereby fixing a liquid crystalline compound on the polymer walls; and PA1 placing the other substrate on the substrate on which the mixture is coated. PA1 providing a mixture between a pair of substrates facing each other, thereby forming a cell, at least one of the substrates being transparent, electrodes being disposed on inside surfaces of the respective substrates, the mixture containing a liquid crystal material, a photopolymerizable compound, photopolymerization initiator, and a radical generating agent; PA1 irradiating light to the mixture to cause a phase separation, thereby obtaining a state in which liquid crystal regions are dispersed in the polymer walls; and PA1 thermally decomposing the radical generating agent by heating the display medium.
However, in the case of method (1), since the liquid crystal contained in the polymer capsule is in the form of an individual sphere, a drive voltage for changing the orientation of the liquid crystal molecules is varied depending upon each liquid crystal region. As a result, a drive voltage for simultaneously operating all of the liquid crystal regions is increased, narrowing the application range of the liquid crystal display device.
In the case of method (2), it is difficult to precisely regulate each diameter of the liquid crystal regions in a spherical shape although a phase separation method is used.
In the case of method (3), it is difficult to precisely align the liquid crystal regions in a round shape in a planar manner although a phase separation method is used.
In the case of method (4), there are advantages in that appropriate resin materials and liquid crystal can be selected over a wide range, since a phase separation is not utilized when the liquid crystal regions are formed; and polymer porous films can sufficiently be purified. However, there are disadvantages in that each diameter of the liquid crystal regions in a round shape cannot sufficiently be regulated; and the liquid crystal regions cannot precisely be positioned in a direction along the surface of the substrate.
In the case of method (5), although the intensity of light scattering is large, it is difficult to uniformly disperse the beads and to cause light scattering at almost the same level in each pixel, resulting in the likelihood of an uneven display.
As described above, in a polymer dispersed liquid crystal display device using polymer type liquid crystal, in which liquid crystal regions are dispersed, it has been difficult to form the liquid crystal display regions in a uniform manner and to precisely regulate the position of the liquid crystal regions in a direction along the surface of the substrate. Thus, the liquid crystal regions have a variety of different diameters and the distribution thereof is not uniform. In addition, because of the difficulty in precisely positioning the liquid crystal regions, the drive voltage for each liquid crystal region is different. This results in a threshold value characteristic curve which is not steep and the drive voltage becomes relatively high. Moreover, a number of small liquid crystal regions having low light scattering ability are present, so that contrast in the resulting display is relatively low.
Since the shape of the liquid crystal regions is not uniform and it is difficult to regulate the position of the liquid crystal regions in a direction along the surface of the substrate, a large screen cannot be obtained under a high precision condition. Moreover, in the case where a simple matrix drive method, in which signals are turned on/off to obtain an average signal and a liquid crystal display device is driven by the average signal, is used for the polymer dispersed liquid crystal display device, a duty ratio (i.e., a ratio of time during which each signal is turned on) cannot be made large.
Furthermore, in the polymer dispersed liquid crystal display device, it is difficult to perform orientation treatment. The reason for this is described as follows:
An example of an orientation treatment method is proposed in Japanese Laid-Open Patent Publication No. 3-52843 and "Liquid crystal", Vol. 5, No. 5, p. 1477, (1989). According to this method, a magnetic field, an electric field, etc. are applied to a liquid crystal display device during a production stage while a polymer is formed by the polymerization. However, in this method, since the surface of the polymer is not directly subjected to an orientation treatment, the orientation regulating ability is weak. In addition, liquid crystal molecules are aligned only in one direction, so that this method cannot be applied to modes such as a TN mode and an STN mode, in which liquid crystal molecules should be aligned in different directions to each other along the facing sides of two substrates sandwiching the liquid crystal.
Another example of an orientation treatment method is described in "Extended Abstracts", p. 320 (The 17th Liquid Crystal Forum). According to this method, liquid crystal molecules are indirectly oriented via polymer walls formed on substrates which are subjected to an orientation treatment. However, in this method, it is impossible to prevent a polymer from remaining on the surface of an orientation film on the pixel electrodes, which makes it difficult to directly align liquid crystal molecules and remarkably decreases an orientation regulating ability in the same way as in the above-mentioned method. This causes serious practical problems in the use of a liquid crystal display device obtained by using this method.
Moreover, as described above, in a liquid crystal display device which uses ferroelectric liquid crystal, requiring polarizing plates and an orientation treatment, a smectic (SmC*) phase is utilized for the purpose of causing spontaneous polarization. The regularity of this phase structure is closer to that of crystal, compared with that of a nematic phase, so that the smectic phase is weak against a physical shock. In order to solve this problem, it is considered that a physical shock is alleviated by dispersing ferroelectric liquid crystal in a polymer. However, this method is not put to practical use since it is difficult to perform an orientation treatment in the polymer.
Japanese Laid-Open Patent Publication Nos. 63-264721 and 264722 propose a method for aligning ferroelectric liquid crystal molecules in a polymer. According to this method, a polymer in which ferroelectric crystal is dispersed is formed on a film and is subjected to uniaxial stretching, whereby the ferroelectric liquid crystal molecules are aligned. However, in this method, since a number of interfaces between liquid crystal regions and the polymer walls are present in one pixel, linearly polarized light which is incident upon a liquid crystal display device is scattered and part of the light is depolarized. As a result, the opaque level of the liquid crystal display device is decreased, which causes deteriorated contrast. In the same way, this problem is caused in other display modes requiring polarizing plates, such as the TN mode, the STN mode, and the electrically controlled birefringence (ECB) mode.
Japanese Laid-Open Patent Publication Nos. 59-201021, 61-205920, and 3-192334 disclose that in order to provide shock resistance of an FLC, polymer walls are formed by photolithography on a substrate material subjected to an orientation treatment to form a cell, and then liquid crystal is injected into the cell. However, according to this method, independent liquid crystal areas cannot be formed and cell thickness cannot be regulated with precision.
As described above, it is difficult to conduct an orientation treatment simultaneously with dispersing a liquid crystal material in a polymer. Even though the orientation treatment can be conducted, contrast is remarkably decreased due to the depolarization caused by light scattered on the interfaces between the liquid crystal and the polymer. The reason why it is difficult to conduct the orientation treatment is that the polymer enters between the substrate and the liquid crystal when the liquid crystal is dispersed in the polymer. The light scattering caused on the interfaces between the liquid crystal and the polymer can be prevented by decreasing the interfaces between the liquid crystal in the pixels and the polymer as much as possible and by making at least one liquid crystal region present in one pixel (i.e., regulating the position and size of the liquid crystal regions). However, at the present time, the liquid crystal regions are naturally formed (i.e., the liquid crystal regions are formed under the condition that the position and size thereof are not regulated). Moreover, a liquid crystal display device using FLC has the problem of low shock resistance as described above.
The liquid crystal display devices obtained by the above-mentioned five methods (1) to (5) are light scattering type devices, and thus, these devices cannot be applied to the non light scattering type liquid crystal display devices of a TN mode, an STN mode, an ECB mode, etc.
In addition, for example, Japanese Laid-Open Patent Publication No. 2-153318 discloses that display areas of a liquid crystal display device are limited in a polymer by using a photomask. According to this method, transparent portions cured by light irradiation and uncured portions covered with the photomask are divided while a voltage is applied between electrodes. Then, the photomask is removed, and the uncured portions are cured to form scattering portions. The display device thus obtained is manufactured in view of a display of an independent pattern. When an electrical field is applied to the device, the light scattering portions become transparent, whereby the entire cell becomes transparent. However, in this method, the shape of liquid crystal is not regulated by the photomask.
Japanese Laid-Open Patent Publication No. 59-226322 discloses that a mixture containing a polymer material and a liquid crystal material is dissolved in a solvent, and the solvent is removed from the obtained solution, whereby a phase separation is conducted between the polymer and the liquid crystal.
Furthermore, Japanese Laid-Open Patent Publication No. 2-116824 discloses a method for fixing liquid crystal regions on polymer walls in a liquid crystal display device. According to this method, a liquid crystal material and a polymerizable liquid crystalline compound having a liquid crystalline functional group attached to its side chain are dissolved in a solvent. Then, the solution thus obtained was coated onto the surface of a substrate. After that, the solvent is removed, whereby a phase separation is conducted between the liquid crystal material and the liquid crystalline polymer to fix the liquid crystal regions on the polymer walls.
In the case of the respective above-mentioned suggested methods, unreacted monomers or oligomers remain in the liquid crystal regions of the polymer dispersed liquid crystal display device. Due to these remaining substances, the viscosity of the liquid crystal is high. As a result, the response speed is low. In order to overcome this problem, Japanese Laid-Open Patent Publication Nos. 4-14015 and 4-168422 disclose the use of a resin material of a fluorine type for the purpose of decreasing the drive voltage and improving the electrical holding ratio. However, when a liquid crystal display device is manufactured by using a resin material of a fluorine type, the response speed .tau..sub.r under an applied voltage is increased due to the presence of the fluorine atoms on the interfaces between the liquid crystal and the resin. In contrast, the response speed .tau..sub.d under no applied voltage is decreased, since the driving force (interaction (orientation regulating ability) between the polymer material and the liquid crystal material) for making the liquid crystal return to the original state is weakened.
Examples of a material generally used as a liquid crystal material include cyanobiphenyl type materials and cyanopyrimidine type materials having a CN group in its molecule. Those materials are disclosed in Japanese Laid-Open Patent Publication Nos. 2-28284, 2-75688, 2-85822, and 2-272422 to 2-272424. However, this CN group is polarized, has strong reactivity, and facilitates the introduction of the impurities of the entire system into the liquid crystal material. Because of this, there is a problem that in the manufacturing process for a polymer dispersed liquid crystal display device which has a number of chances to come into contact with other compounds, the liquid crystal display device thus obtained cannot maintain a high electrical holding ratio (90% or more). Moreover, in the case where the polymerizable material contained in a mixture of the liquid crystal material and the polymerizable material is cured to cause a phase separation between the liquid crystal and the polymer, reactive sites of the liquid crystal and the polymerizable material coexist in the mixture, which damages the liquid crystal to remarkably decrease the electrical holding ratio.
Furthermore, in a method for causing a phase separation between the liquid crystal and the photosetting resin by curing the photosetting resin, it is easy to regulate the size of liquid crystal regions; however, an unreacted monomer remains in a display medium containing the liquid crystal and the resin, and the strength of the polymer walls formed of the resin is not sufficient. Thus, electro-optic characteristics of the obtained cell is varied due to thermal change. In addition, there is a problem in that the adhesion between the substrate and the polymer dispersed liquid crystal material is low, so that the polymer dispersed liquid crystal material is partially peeled off from the substrate due to the contraction of the resin.